Fix LibMyPaint building issues on Windows

This MR fixes the following issues I found on Windows:

• Add configuration-based library lookup. @rempt patched in a CMake toolchain, so I tacked the rest of the missing logic (library name, dependencies, and version)
• Standardize configuration lookup. The existing module used a mix of "LIBMYPAINT", "LibMyPaint", and "libmypaint" as library names.
• Support pkg-config correctly. If installed from 3rdparty on Linux, the CMake toolchain did not install the .pc file -- but the Find module would still try to look it up, and thus it'd fail to find the library version, making it fail the lookup whole. I tackled this along with the configuration files for Windows.
• Several classes were labeled as PAINTOP_EXPORT -- which, on Windows, marks them as dllimport, not dllexport (we're using libpaintop here, not declaring it).

Test Plan

Build Krita on Windows from scratch.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.