Fix various compiler warnings and pass --werror to `meson setup` to prevent them sneaking in in future

See the commit messages.

assigned to @pwithnall

added 2 commits

• d1736db4 - ci: Use modern meson subcommands to setup and compile
• f1fe1773 - ci: Pass --werror to `meson setup` to catch compiler warnings

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added 9 commits

• 83847839 - xdgmimecache: Fix a signed/unsigned integer comparison warning
• e4944119 - xdgmimecache: Fix unused parameter warning
• fc109b01 - test-mime-data: Explicitly mark switch fallthroughs
• a806522c - src: Mark some unused parameters
• d6d5987b - ci: Use modern meson subcommands to setup and compile
• 26994986 - ci: Pass --werror to `meson setup` to catch compiler warnings
• 14a32bd8 - build: Enable -Werror=sign-compare
• 45551489 - build: Enable warning_level=3 for Meson
• 9ce3ed6b - build: Add a selection of stricter compiler warnings

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mentioned in merge request !34 (merged)

approved this merge request

resolved all threads

merged