fix: open file dialogs in a sensible default directory instead of the working directory

[BenJule]

Several wxFileDialog call sites passed an empty directory, so they opened in the current working directory (which can be a temp folder) instead of somewhere useful.

Relocation dialogs (a file path is known) now open in that file's own folder, falling back to the last used directory:

• "Select a new file" (replace a volume's source file)
• "Please select a file" (locate a missing input on project load)

Open dialogs (no prior path) now open in the last used directory:

• Select a G-code file (GUI_App)
• Import bed shape / texture / model (BedShapeDialog)
• Select images (StatusPanel)
• Open CA certificate file (PhysicalPrinterDialog)

No functional changes — only the dialogs' initial directory. Adjacent to the temp-working-dir behaviour reported in #11253, though it does not by itself fix the Import-3mf / Save-As default there.

[Haidiye00]

Hi, please merge it into one patch and then submit your test results. Thanks.

[BenJule]

Done - squashed into a single commit.

On the change itself: all the affected dialogs (open/import/export/replace across BedShapeDialog, PhysicalPrinterDialog, StatusPanel, Plater and GUI_App) now start in a sensible folder - the referenced file's own directory when there is one, otherwise the last-used directory - instead of the process working directory. The behaviour to check is: open one of these dialogs, pick a file in some folder, reopen the same dialog and confirm it starts in that folder again rather than the CWD.

Full disclosure: I don't have a macOS/Windows GUI build environment here, so I've verified the logic by tracing the code paths rather than capturing a recorded run. If a maintainer build is handy, those are the flows to confirm.