Rewrite the share dialog in QML

[claucambra]

We have several open issues from the design/server/etc. side regarding desired changes to sharing, or the share dialog itself, open in this repo:

• Update Share design #3109
• Enhance sharing #3085
• Combine sharing / activity as "file detail" #3845
• Graphical calendar when setting the expiration date #3954

Two of these, particularly #3109 and #3845, are currently not doable due to the share dialog being a QWidget rather than a QML component. Other benefits of QMLification would include:

• Better integration with the tray window
• Reuse of existing components used in the tray window
• Far easier visual customisation of components
• Easier extension and addition of new components

The least painful solution would be to rewrite the share dialog in QML -- this would be a bigger task up front to reimplement everything but would pay dividends once this was done.

@mgallien @tobiasKaminsky @jancborchardt

[tobiasKaminsky]

This then would look more like tray window, but not any longer OS specific, right?
@jancborchardt I remember that we wanted to stay close to OR look&feel, is this still the case?

[claucambra]

@tobiasKaminsky not necessarily, we can make it look any way we want (including native)

[tobiasKaminsky]

I added it to board.
Once it is picked up, we can discuss actual UX and track the subtasks.

[jancborchardt]

@claucambra cool, sounds good. Native look is definitely nicer in the long run for that.