cxx-qt-build: return an Interface from CxxQtBuilder

[LeonMatthesKDAB]

Return an Interface from the CxxQtBuilder.

This is a rebased version of the original #1224 by @ahayzen-kdab

• cxx-qt-build: split interface into a separate file
• cxx-qt-build: move write_manifest to export of Interface
• cxx-qt-build: move write of exported include directories to interface
• cxx-qt-build: store manifest and dependencies in Interface for now
• cxx-qt-build: return an Interface rather than taking as input
• cxx-qt-build: check the links key when exporting
• cxx-qt-build: split include and exported include dirs
• cxx-qt-build: include headers automatically like CXX
• cxx-qt-build: remove CxxQtBuilder::library instead use new and export

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4979207) to head (45162f0).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1285   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        12772     12780    +8     
=========================================
+ Hits         12772     12780    +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.