Removed PySide support from common.py
Changed buttons to QDialogButtonBox in NewTaskDialog
Cleaned up UI creation in TaskWidget
Minor stuff
Changed desc_input to multiline in NewTaskDialog
Refactored UI code, made NewTaskDialog permanent*

The UI in mnemosyn.py is now somewhat understandable and reusable.
NewTaskDialog is no longer destroyed/recreated whenever opened or closed,
sinced PyQt is kinda memory-leaky when it comes to removing stuff.
Added tags and a taglist-widget