gtk3: convert single-hit files as a trial for the GTK2→GTK3 migration

[chrisdebian]

Summary

Per the migration scoping on #13 and your go-ahead there ("Please go ahead since nobody showed up lately. Your plan sounds good."), this is the trial PR: the three lowest-risk "single hit" files, proving the conversion pattern and the dummy-rig testing rhythm before touching anything larger or drawing-related.

• rig-gui-buttons.c, rig-gui-ctrl2.c, rig-gui-func.c — each gtk_vbox_new()/gtk_hbox_new() call gets a GTK_CHECK_VERSION(3,0,0) branch to gtk_box_new() with explicit orientation, so the same source builds under both GTK2 and GTK3.
• rig-gui-func.c also needed a second fix the original deprecated-API grep missed: GTK_DIALOG(dialog)->vbox is direct struct access, removed in GTK3. Replaced with gtk_dialog_get_content_area(), available since GTK 2.14 — below this project's existing 2.24.0 floor, so no version branch is needed for that part.

configure.ac's GTK3 pin is deliberately not part of this PR — once this pattern's proven, that's the next step.

Test plan

• Full build succeeds clean under the current GTK2 config (only pre-existing, unrelated deprecation warnings — nothing new)
• All three files individually compile clean against real GTK 3.24 headers (checked by temporarily pointing the build at gtk+-3.0 locally, not committed — the rest of the codebase isn't converted yet, so a full GTK3 build isn't expected to succeed until later phases)
• Ran against Hamlib's dummy rig backend — main window, RX Levels, TX Levels, and the Special Functions dialog (the one with the content-area fix) all open and render correctly, controls laid out the same as before

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