Change SVG export dialog to support bitmap exports w/ PPI

[cmeyer]

Fixes #1852

• Add export_bitmap_file function; allow font-size override.
• Change SVG export dialog to support bitmap exports w/ PPI.

AI assisted (dialog).

This is a step to update the existing dialog; follow-ups will consolidate the various export workflows, make exporting modeless (and threaded), etc.

Note: the font size is fixed at 1/6 PPI in this version. PPI for SVG is 96.

The menu item is renamed from Export SVG... to Export SVG/Bitmap... in this PR.

[MattRoyle]

LGTM

[Ion-e]

Issue and special mentions:

1. No persistence in default selection (Consistently), so making several exports of the same size can be tedious. The only attributes that remain persistent are "SVG/Bitmap image", and "PNG/BMP/JPEG", all other ones reset to default values.
2. When exporting to SVG, the "File format" drop down remains shown, which is misleading as it states the format will be either PNG/JPEG/BMP. There is an inconsistent use in the UI regarding the drop down enable/disable functionality. A disabled drop down should indicate that the values are not editable, not that the values are not going to be used. This behaviour is inconsistent in this same window between the aforementioned File Format drop down and the PPI drop down values.
3. Sometimes when swapping from Bitmap to SVG options, you have to click twice which makes me think the cursor loses focus.
4. It's not clear UI to have the "Custom" option as an option in the "PPI", and then having 2 PPI edit boxes. This should be reworked.
5. The "Units" expander is not intuitive to be referring to "Width/Height", the UI should be reworked to reflect this, potentially placing the units first as a user is more likely to first select the desired units then select the unit values as standard workflow.
6. Units drop down and PPI dropdown have different lengths.
7. Sometimes, when swapping from SVG to Bitmap, the PPI drop down doesn't become enabled automatically, the user has to manually change the Units drop down for it to become enabled.
8. Export as and File format dropdowns could be merged to "Export as: SVG/BItmap PNG/Bitmap BMP/Bitmap JPEG"
9. We could change the "Custom" option in the drop down for aspect ratio by adding the Superscan Width/Height button:

10. How were the Default PPI values chosen?
11. The custom PPI edit box and PPI drop down could be merged into 1.
12. The "Default (5:3)" can be changed to "(5:3)".

After this issues have been addressed, a new round of testing should be done on this feature. Thanks