mainwin: Add support for shift+dblClick to insert syllable or chord

[matevz]

Part of #84:

• shift+double click now prepends new syllable, chordname, function mark or figure bass mark in front of the element
• also, if syllable, chordname or figured bass mark button is selected (Insert mode) the element is prepended in the same way (to edit an existing element click / double-click on it)
• refactors repositSyllables(), repositFunctions etc. to common repositionElements which is now virutal in CAContext
• refactors addEmptySyllable(), addEmptyFunctionMark() etc. to a common addEmptyElement which is now virtual in CAContext
• adds clickTimerActivated() to CAScoreView so we can read whether mouse press can be part of doubleclick or tripleclick before doing any other actions in mainwin
• does not hide the Insert toolbar anymore when in Insert mode. This may have caused that the user could not untoggle the button and exit the insert mode
• fixes score view vertical shifting for 2 pixels on most desktops when QTextEdit is shown in the top toolbar or not

[suamor]

I checked and this is old code, but was / is there a plan to add more code?
Else for loop could be removed.

[matevz]

Actually, the for loop here is to compute the value of i (it's declared outside of the for block), because we have no more intelligent way of computing the position than linearly scanning the list. We could do binary search in the future though.

[suamor]

Addedd one comment about old code, that could be optimized.

[suamor]

We should make use of Unit Test generation features from github.