Asset editor improvements

[fredsa]

• Reformat affected files prior to other commits
• Fix asset undo (Asset Editor doesn't undo properly #213)
• Implement asset editor redo
• Create distinct undo for each asset edit
• Limit asset editor to undos in current session
• Fix stacking "no more undo/redo" alerts
• Ignore ';;' that is a new ';;{…};;' asset block
• Asset editor displays line number ranges and header
• Click line number range to jump to highlighted lines
• Show errors for assets with incorrect number of values
• Fix bad edits when source has different length values
• Fix length new data values written
• Align thumbnails to the top to prevent visual jumps
• Dismiss highlighted lines on user interaction
• Replace affected asset range, not entire file contents
• Asset undo inside source editor shows affected lines
• Likely fixes [Possible bug] Asset editor gets reset #62

[fredsa]

Updated PR title and the initial comment

[fredsa]

Rebased commits on origin/master

[fredsa]

Last two commits add:

• affordance for asset line numbers in asset editor
• clickable asset editor decorations in source code

[fredsa]

Rebased on origin/master

[fredsa]

• Fix validateAssetData, add max reachable offset
  calculation, fixes NES issues with chr_generic.s
• Add validateAssetData support for rletag
• Fix broken asset editing and undo when asset source
  has not been previously opened manually, for example
  when reloading the #asseteditor page directly
• Fix palette color undo/redo

[fredsa]

• Implement #asseteditor/filename/lineno support,
  and highlight selected asset on navigation

[fredsa]

• place the "asset editor" decoration after header instead at the end of the line