Polishing

- Clear the content text box and select it after a paste has been made.
- Removed username check, it is not possible to have no username now.
- Added default username (Paster) and colored it gray when it is saved as
  the default.
- Removed FontAwesome comment reference
- Moved Main_Activated code to Main_Load
- Moved textBoxUsername_TextChanged code to textBoxUsername_Leave

Signed-off-by: Steven Downer <grinch@outlook.com>
Implement history function

Signed-off-by: Steven Downer <grinch@outlook.com>