Add Undo/Redo to InputTextWidget

[Schlumpf]

Additional small improvements:

• Disable Actions if they can't used (i.e. nothing to copy)
• Don't allow to copy an empty sting
• Add icons
• Add Placeholder text

[nurupo]

The placeholder was added to QTextEdit in Qt 5.2, which is planned for November (alpha at the end of September), so this placeholder code probably would be just temporary.

[nurupo]

Also, do icons for undo and redo look so bad that you decided on adding icons only half way through? : )

[nurupo]

Some comments on formatting. Use curly braces for one-liners, don't move the first curly brace on the new line, and put spaces between keyword, condition and the first curly brace. In summery:

if (bool) {
  something;
}

For method definitions, class definitions, etc. the first curly brace goes on the new line.

[nurupo]

and put spaces between keyword, condition and the first curly brace

if<space>(condition)<space>{. Same applies to for, while, etc.

if (condition) {
  statement;
} else {
  statement;
}

[nurupo]

I accidentally deleted commit in which I merged your animation pull request and couldn't restore it, so I decided to merge this pull request, since it contained the things I accidentally deleted.