chore: cleanup, document, and test some RTL code

[Nerixyz]

This PR is an attempt at cleaning up the RTL code and documentation a bit so it's easier to understand (that's subjective, I know).

• Renamed containsRTL to lineContainsRTL_ and checked for lineContainsRTL_ || isRTL() in breakLine.
  This way, a few RTL words in a wrapped LTR message won't affect the lines below.
• Renamed first and FirstWord to textDirection_ and Direction respectively.
  That's what they are about. first also "leaks" how it's detected (finding the first word that has a direction).
• Added isRTL, isLTR, and isNeutral to check the text direction.
• Moved from int to size_t and qsizetype (for signed values).
• Added some assertions in addElement to check for weird usages of elements.
• Moved from std containers (vector/stack) to QVarLengthArray.
  Single lines usually have very few elements, so we can store them on the stack (if that's not enough, the container will allocate).
• Moved wasPrevReversed_ from a member to a local variable (isReversing)
• Updated documentation for addElement and reorderRTL (and the other stuff I added)
• Added basic tests for reordering.
  The tests only test the reordering on a single line. The line must have a username element. They're still a bit WIP, as e.g. images and mentions aren't tested.

[Nerixyz]

Not worth another PR imo.