Double-click to select word when editing text in score not implemented

In most contexts double-clicking on a word will select that word. This doesn't happen when editing text elements on the score, e.g. titles, composers, tempo text, lyrics. For some users - certainly this one - double-clicking to select a word is a deeply ingrained habit and so the absence of the double-click to select a word is quite noticeable. For users that don't have this habit, it won't be an issue at all.

I've created a PR (https://github.com/musescore/MuseScore/pull/5101) that implements double-click to select a word when editing text in the score. At the same time the PR tidies / refines some of the other text editing UI in this context:

• makes the highlight match the highlight colour used in other contexts in the MuseScore interface (colour and being slightly bigger than the highlighted text)

• makes the cursor a bit thiner so it's more like the cursor in other context

• hides the cursor when text is selected, so it's more like the cursor in other contexts

• changes the appearance of the mouse point to a I-beam when the mouse pointer is in a text element that is being edited

• makes it so a second click on an editable text element puts it into editing mode - the existing "double-click" to enter editing mode remains and a second click that begins a drags still does the same as before, i.e. moves the element - having the second click work like this is very common and, I'd suggest, is expected behaviour.