Prepare KDb for release wrt versioning, dependencies and docs

Summary:

• Update README-PACKAGERS doc to reflect naming and notes about compatibility + we no longer use and ship .desktop files

• PC file: add explicit Qt5Widgets deps, library -> framework

• Bump soname to separate from incompatible 3.0 branch

• adds PROJECT_STABLE_VERSION_MAJOR to the lib name
• sets VERSION to PROJECT_STABLE_VERSION_MAJOR.PROJECT_STABLE_VERSION_MINOR.PROJECT_STABLE_VERSION_PATCH
• sets SOVERSION to PROJECT_STABLE_VERSION_MAJOR (special case for 3.0 < PROJECT_STABLE_VERSION < 4.0: sets SOVERSION to PROJECT_STABLE_VERSION_MAJOR + 1 to separate from incompatible version 3.0)

This is a template for other KEXI libs too.

Test Plan: build+install KDb

Reviewers: piggz, pino

Reviewed By: piggz

Tags: KDb

Differential Revision: https://phabricator.kde.org/D10254