Implement special character menu on long key press

[quassy]

It's very unfortunate that writing certain typographic symbols (e.g. quotation marks, long dashes), special characters (arrows), language-specific letters (ß, φ, ñ; unless you change the keyboard layout) and emojis is so hard on elementary compared to for example Android and macOS.

The latter has a great feature called "Character Accent Menu" which basically adopts the way touchscreen keyboards work: Long press a key and a menu pops up showing alternative characters, which can be selected by typing a number. Considering how rarely one needs to repeat letters on end in normal writing & code, a similar (optional) feature would be a great addition for elementary.

[Maxim Taranov (png2378)]

We can enable typing of additional typographic symbols by adding 'misc:typo' value on 'org.gnome.desktop.input-sources xkb-options' path via gsettings or dconf-editor. Also we need to add key for choose 3rd level (for example Right Alt - 'lv3:ralt_switch'). As a result, the keyboard layout would look like this: https://launchpadlibrarian.net/266996930/Russian_keyboard_layout.png

This method works fine for me on Russian and English keyboard layouts, but there can be problems with layouts with native 3rd level (such as German or Spanish). For this reason, this option was not included in switchboard-keyboard-plug. OP, you can ask @santileortiz in Slack for more info.

I am sure this option is necessary, but we need ideas how to implement it.

[quassy (quassy)]

The problem with extra levels or alt codes is they are neither flexible (think about all the emojis that get added almost daily, or people wanting to type the correct typgraphic symbols for multiple languages without having to switch the keyboard layout everytime) nor discoverable, which is exactly why I'm proposing something different. Stuff like typing AltGr+- (or Alt+0150 on Windows) to get a half em dash ("–" common in German) only works for a limited number of characters, because space is limited and users can't know / remember hundreds of combinations not actually labelled on the keyboard.