add double click for text and lyrics manipulation, other improvements [sf#27]

[matevz]

Reported by matevz on 2018-04-16 22:21 UTC
Steps to reproduce:

1. Add a note
2. Add text sign on top of the note
3. Select a text sign (without changing it) - e.g. to delete it. Since we merged the edit and view mode, it became impossible to just select an element without entering the text mode. The same behavior should be for lyrics syllable and chord names.

Suggested solution:

• Implement single click to select an element, double click to enter the text mode.
• In light of staff name, voice name and other text widgets not updated when deselecting #114, leaving text mode should always store the text. If a user wants to cancel the change, he can always Undo.
• When empty text, bookmark, syllable and chordname element is selected, render it with a single red dot, so a user knows there is something in the selection. layout: Render placeholder if text/bookmark empty #195
• Now that we can select a text-like object, delete can just clear the syllable or chord and shift+delete should remove the element + shift the following elements back.
• Similarly, when Insert syllable or chord name buttons are pressed, replace the existing chords. If shift is pressed, insert a new chord right before the selected element. The same if you do shift+double click. mainwin: Add support for shift+dblClick to insert syllable or chord #204

Potential issue:

• For lyrics and chord names, this removes the select-all-in-measure feature for double-click and selects all in context on triple-click. We could overcome this, by activating select-all-in- feature, if you clicked between the elements in an empty space.