Issue2795: Add shortcut setting and handler for "Activate Fit to window" feature

[RonnyFenrich]

This PR implements my own feature request :)
It allows the user to define a shortcut which will toggle the "Fit to window" player functionality.

• Adds a new entry to Preferences > Shortcuts > Player: Activate Fit to window
• Adds shortcut_activate_fit_to_window shortcut handler triggered by that new shortcut key which will just trigger a click of the existing 'Fit to window' player button

[ImprovedTube]

hi @RonnyFenrich yay! :)
nice!

It is requiring to have the button enabled. Some users enabling the shortcut won't know about it, so to consider them, you could make the shortcut only trigger the function, or enable and disable the button too each time, if the button was not already enabled in our menu at the time of setting the shortcut.

We also have the same feature at appearance : player : player size: max. height* and other options like max. width, which you might like ( #2794 (comment) )

Generally we can also make shortcuts change one of the toggles in our menu too, by changing chromium storage variables.

youtube/js&css/web-accessible/www.youtube.com/shortcuts.js

Lines 460 to 467 in 75bfa7e

	4.7.25 DARK THEME
	------------------------------------------------------------------------------*/
	ImprovedTube.shortcutDarkTheme = function () {
	if (document.documentElement.hasAttribute('dark')) {
	// message will propagate all the way to setTheme() so we dont need to do anything more here
	ImprovedTube.messages.send({action: 'set', key: 'theme', value: 'light'});
	} else {
	ImprovedTube.messages.send({action: 'set', key: 'theme', value: 'dark'});

CSS features and most JS features are prepared to react once any of these variables changes, including the player button "fit-to-window"

youtube/js&css/web-accessible/core.js

Lines 207 to 213 in 75bfa7e

	// REACTION OR VISUAL FEEDBACK WHEN THE USER CHANGES A SETTING (already automated for our CSS features):
	} else if (message.action === 'storage-changed') {
	let camelized_key = message.camelizedKey;
	ImprovedTube.storage[message.key] = message.value;
	if (['block_vp9', 'block_h264', 'block_av1'].includes(message.key)) {
	let atlas = {block_vp9:'vp9|vp09', block_h264:'avc1', block_av1:'av01'}

) ...

---

*So the player button "fit to window" actually is "max height" and the icon's arrow will more accurately be vertical, instead of should not be diagonal. And this project should be about keeping all these things consistent ( deduplicated )