Refactoring of TTS

[di72nn]

A separate PR for TTS. Based on #931.

I tried to improve code and fix potential synchronization issues while keeping the logic mostly intact (except some initialization stages - those had to be changed because TTS didn't work for me).
It is still far from perfect, but that should do for now.

So far I only tested it a bit with Google TTS on an Android 9 emulator and a LineageOS 16 device.

I may make some more improvements, but no promises.

Outstanding issues (in general, not in this PR):

• Bad handling of interruptions (and pause/resume in general). Discussed in After interruption, TTS restarts at wrong place #939.
• Reset to the start of the article on interruptions (After interruption, TTS restarts at wrong place #939). As @tyndare mentioned: resume will check that the current text unit is visible on the screen, if it is not the case it will resume at the first text unit visible on the screen.
• The size of "text units" should be reduced.
• I'm quite annoyed that text in inline tags is pronounced as separate sentences (e.g. there are pauses around links).
• Splitting into sentences is dumb: it will split stuff like the U.S. and (it basically splits by any ., ?, ! followed by a space).

@tyndare here are the latest changes in case you want to give it a go. Please comment if you do.

[di72nn]

Inline elements are no longer separate text units.
Text is split into sentences (not perfect, see the first post).

[di72nn]

Add rudimentary "emphasizing" for bold/strong/italic text. I didn't find any way to make Android TTS emphasize parts of text, so I just add commas around these emphasized parts to denote them with pauses. I'm not happy with the amount of code (and its complexity) it takes, but oh well.

Also made TTS pronounce alt text or title of images.

[di72nn]

Don't reset reading position if scrolling didn't change since pause (fixes #939 as a third option described here).

Fixed TTS notification icon: a wallabag logo is now displayed instead of a solid square.
Notification action icons are replaced with better looking ones (without outline).