Replace obsolete functions

[YomikoR]

Prerequisite for porting to Qt6. Regression may happen.

Things that would affect building with Qt5:

• trUtf8 -> tr
• QKeySequence operator + -> |
• QLabel::pixmap() will return by value -> don't use it
• QProcess::start() args -> dirty fix by appending empty arg list
• misc changes for texts (won't compile with an older version e.g. Qt 5.6)

Things that could be supported by Qt5Compat:
- [ ] QRegExp -> QRegularExpression (wildcard matching?) QRegularExpression is not an exact replacement as of today.

Things that doesn't exist in Qt6:

• QtWinExtras -> (?) This was used to show progress animation on the icon in task bar. Maybe it will be added back in Qt 6.3.