Replace deprecated QSignalMapper by lambda expressions

Summary:

• QSignalMapper can mostly be replaced using straightforward lambdas, reducing the code size

Test Plan:

• switch scope and arrangement of problems in ProblemView
• create an application using the AppWizard
• add a compiler
• switch representation of register variables
• add code to an existing file using a file template
• open a file with an external application
• switch to header/source file using the context menu action
• track changes of a variable in the debugger
• status bar seems to work
• untested: runcontroller

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9881