feat: multi-click selections

[Kneemund]

Adds multi-click selection within typewriter text fields, mimicking the behavior of most text editors/browsers.

• Fixes Double click to select word and sentences in the text? #872.
• Double click to select the current word. Drag after the second click to extend the selection onto other words.
• Triple click to select the current line. Drag after the third click to extend the selection onto other lines.
• Cycles through selection modes (caret, word, line) in case of more than 3 clicks.

Things to Discuss

• Performing a double click selection while hovering over whitespace will select the next word, but immediately expand the selection backwards onto the previous word (i.e. cover two words). This is nearly identical to the behavior of Firefox; the only difference being that Firefox only expands the selection after the mouse is moved in any way. GNOME Text Editor simply selects the whitespace because it generally snaps the selection to any word bound (start and end). I'd personally prefer the way GNOME Text Editor handles this.
• The caret blink is always reset if the mouse moves, even if the selection doesn't change. This has been the case before but is more noticeable with multi-click selections, since the selections change less often. Not sure what the intended behavior is.
• The gesture used for detecting multiple button presses is currently always enabled and has a generic name. It could either be tailored to this feature specifically and disabled when the typewriter tool is not selected, or an early return could be added (keeping it generic enough for potential future features that make use of the gesture).