This is a completely normal condition when running in the AppImage
container, so it just amounts to unnecessary logspam.

Don't log attempt to set font, just failure
Don't log when setting dark theme
Convert "Processing command-line arguments:" into debug message

Don't log confusing "Skipping English..." message
Don't log _attempts_ to load translators
Log successful attempts as debug messages

Don't always log versions from project file in `upgrade_project_data_structures()`, only log when upgrade paths are taken.
Don't spew generic message whenever files contain temp blender paths.
Updated file-recovery output to be more informative
Replaced some %-formatted strings in log messages with .format() strings

Don't log every time the preview is resized
Promote "Error applying JSON to timeline..." messages from info to error

This will allow some log messages in Angular source to be reduced from
`log.info()` to `log.debug()` priority

This reverts commit 3f453de.

Nearly every message is reduced from 'info' to 'debug', some
redundant/low-information logs are eliminated, and a new, terse
(but actually more informative) standard formatting for messages
is introduced and applied to as many log messages as possible.

* Deleted "flow" checkpoints which logged no data (e.g.)

   ```python3
   log.info('currentChanged')
   log.info('updateSelection')
   ```

* Reduced most other messages to `log.debug()`

* Deleted _numerous_ blocks of program-flow debug printfs, like:

   ```python3
   if result == QDialog.Accepted:
       log.info('Import image sequence add confirmed')
   else:
       log.info('Import image sequence add cancelled')

   # Or, every time a callback fires...
   log.info('actionRedo_trigger')
   ```

* Many "checkpoint" messages removed or reduced to `log.debug`
   * Where possible, checkpoint messages that logged every time
     through a loop were replaced with more informative messages
     that only log under interesting conditions, letting the
     uninteresting loop iterations pass silently

* Remaining useful `log.info()` messages formatted to contain more
  information, log more tersely (sometimes combining two or three
  consecutive messages into a single, concise form

* Made `keyPressEvent` log messages debug-only

I got a little too ambitious beefing up one of the messages in
`properties_model.py`

Every message is reduced to `log.debug()` _except_ the one about
changing the debug mode.

All debug messages are formatted with useful context, so...

```python3
log.info(value)
log.debug('{} set to {}'.format(param["setting"], value))
```

All `qt_log()` messages in the angular code are replaced with
`qt_debug()` which only logs in debug mode.

This commit enables something I've been working towards for some
time now: The ability to configure OpenShot's console output and
logfile output to different logging levels, with potentially _more_
messages being logged to the logfile than are shown on the console.

As a result, it brings all of the following:
* Log messages of level INFO or higher are written to the console
  log
* By default, initially the same messages are written to the
  logfile as well
* The `debug-mode` preference now affects openshot-qt's own logging,
  as well as the libopenshot proxy logging. When `debug-mode` is
  switched on, the `openshot-qt.log` file will receive all messages
  of level DEBUG or higher.
* Log messages proxied from libopenshot are now logged at level
  DEBUG, instead of INFO, so that they will be written to the
  log file (only if `debug-mode` is on), but will not be printed
  to the console output.

This opens up the possibility of adding a second ZeroMQ logging
channel for messages at a _higher_ severity than debug, which
would always be active rather than being gated by the `debug-mode`
switch. That priority logging channel could be used for messages
which openshot should always log, such as error messages. This
would be a possible means of addressing OpenShot#1450.