Fix electron context menu copy/save-as

[t3chguy]

Fixes element-hq/element-web#13845

We had a bunch of issues with the copy/save context menus we presented. The above being due to the showSaveDialog going async, instead of using the new showSaveDialogSync option I opted to switch it and the fs calls to async/await.

• fixes save-as: awaits showSaveDialog
• fixes save-as of data:// urls: by extracting the buffer from the nativeImage
• hides the save-as option for blob: urls due to them only existing in the renderer process
• uses fs.promises.writeFile instead of writeFileSync
• cleans up the code a little
• hides context menu for in-app icons like:
  

I tried to (ab)use the clipboard for save-as on blob URLs and whilst it did work fundamentally, there was no way to accurately get the timing right. There is no way of knowing when copyImageAt is done and the clipboard is ready for us to read the image from.