Use Qt 5.2.0's placeholder text implementation for Input Text widget

[nurupo]

Qt 5.2.0 will be released near the end of this year with a text placeholder implementation for QTextEdit. Since the placeholder text implementation in Input Text widget is a little buggy — it changes the QTextEdit's text, which affects change history (the placeholder shows up in undo/redo), instead of drawing a placeholder text as a separate from the actual text entity, as Qt does — we could use Qt's one.

[Schlumpf]

assign me to this issue, please. I'll do it if 5.2 is released.

[nurupo]

I though I could fix it, since it will be pretty fast, there is just one line that should be added to set a placeholder with Qt 5.2.0 (and several more removed).

Oh, and I will have to provide Qt 5.2 for travis, which is not a big deal.

I can assign issues only to people who have commit rights to the repo, which you have not, since I still want to check your code through pull requests before merging :)

[Schlumpf]

ok