Improve handling of images from the clipboard, and drag&drop

[jmbreuer]

Description of requested feature:

Using Edit/Paste (or more commonly, Ctrl+V) on Shutter's main window should add the image on the clipboard to Shutter's session (adding a new tab with that image, that's how I'm used to the session interface to work - I'm not sure if it can also be configured differently). I've seen - after some digging - that Ctrl+Shift+V is already supported, but quite tucked away. Which is great to know for my current workflow, anyway - but please take the rest of this into consideration, as well.

Also, the notification icon menu could have an "Add Image from Clipboard" option (active or maybe even visible only when there actually is image data on the clipboard).

Furthermore, both the main window and the notification icon should be drop targets for images, so that this same functionality can also be accessed by dragging & dropping images into/onto Shutter.

Reasons for adding feature:

Especially when putting together documentation, I find that I often want to use existing images from web sites, instant messages, ... intermixed with 'proper' screen captures - possibly editing/cropping/etc those as I want to use them in the final document.

Shutter's session management / tab interface plus image editor already are a very viable "light table" for this use case, except that I thought images from elsewhere had to be saved somewhere, and could be imported with File/Open (Ctrl+O) only.

Extra information, such as Shutter version, operating system and ideas for how to implement:

I noticed some 'buggy' differences in handling between images imported from clipboard and other types, I've put those together in #595.

Shutter 0.99.3/Rev. 1607 on Gentoo Linux, KDE Plasma 5.27.5.

[Photon89]

Also, the notification icon menu could have an "Add Image from Clipboard" option (active or maybe even visible only when there actually is image data on the clipboard).

Shutter has quite many useful features, but eventually, only some of them are accessible through the tray icon. That's mostly in order to not clutter the menu with too many entries and make it overloaded. I am pretty sure that for most of Shutter's options there are users who'd benefit from adding the options to the tray menu, but if we add all options into that menu, it will become a chaos...

Furthermore, both the main window and the notification icon should be drop targets for images, so that this same functionality can also be accessed by dragging & dropping images into/onto Shutter.

That's a good idea! Don't know, how tough to implement, though...