Launcher project changes

[iLLiCiTiT]

Issue

Project changes may not be propagated in right way. So project clicked on first launcher page may be propagated to project combobox on page 2 but is not propagated to assets widget which is confusing and may cause crash of OpenPype Tray.

This happens since #1725 .

Changes

• implemented ProjectHandler which handles project changes and refreshing of projects
• all callbacks related to project changes does not care about anything else that value passed with project change signal
  • ProjectHandler object always make all necessary steps before callbacks are triggered
  • this simplified a lot of code as propagation of project change was sequence based and race coditions were not always right