Win32: Refactor, split off and fix resize logic and DPI changes handling

[prokopyl]

This PR refactors and splits off the logic that handles re-sizing and discovering/updating the window's DPI.

Some of that logic is split off into a new, low-level Win32Window type, which only handles translating baseview types and assumptions into Win32 calls, separately from managing events or the window handler.

This is part of the effort to split up #174 into smaller pieces.

This PR actually changes the logic, mainly by also splitting the internal window_info into two separate components, current_size (physical) and current_scale_factor, which are instead updated independently during their respective events.

Incidentally, this PR also fixes a few bugs:

• Fix the window handler not being notified of the new scaling factor that's discovered right after the window is created.
• Fix a WM_DPICHANGE event triggering a window resize but not triggering a resize event in the window handler. This supersedes the work done in Fix WM_DPICHANGED not triggering Resized event #140.
• Fix handling of the WM_DPICHANGE event to use the OS-provided window dimensions and position, instead of computing our own dimensions and leaving the position as-is. This avoids a DPI-change loop due to the window constantly switching between two monitors as it's being dragged across them using the mouse.

This has all been tested on my Windows 10 machine and works as expected.