[Palette Editor] Add Images to Importing/Exporting, Bugfixes

[Lavesiime]

This Pull Request adds two features to the Palette Editor:

• Added the ability to import the palette from an indexed image when importing palette colours. Closes [Palette/Scene/GameConfig Editor] Allow palette exporting/importing from image files #174.
• When exporting a palette, the user can now save the palette into an existing GIF image.

Both features are usable in all versions of the Palette Editor, from the standalone version to the GameConfig version to the Scene Editor version.

Additionally, some minor Palette Editor bugfixes are included as well:

• Fix importing from v4 GameConfigs and pre-v5 StageConfigs. Previously, the editor would crash because it never actually loaded the selected file before attempting to use its data.
• Add a maximum cap of 256 colours when loading from an .act palette file during the Import Palette process. Previously, the editor would allow more than 256 colours to be loaded, but any past the 256th would be inaccessible and only partially visible.
• Fix the Export button becoming permanently disabled after opening an .act file, even when then opening a different palette file type.
  • Instead, the Export button is never disabled in the first place, so that all palette types can utilise the previously mentioned exporting feature.