[mini-browser] fix 'Open Source' implementation

[vince-fugnitto]

Description

Currently, the implementation of the mini-browser openSource may be flawed for certain file extensions.

Here is the current implementation of the openSource.
https://github.com/theia-ide/theia/blob/730afb49ffb4be2f1b839bfce364fa0e938034f6/packages/mini-browser/src/browser/mini-browser-open-handler.ts#L236-L243

The call open calls the following opener-service method:

https://github.com/theia-ide/theia/blob/730afb49ffb4be2f1b839bfce364fa0e938034f6/packages/core/src/browser/opener-service.ts#L80-L83

Based on the implementation of getOpener, the handler returned has the highest priority.

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In the mini-browser we have both the Open Preview and Open Source toolbar items, used to open the uri in question in two different modes. The problem with the Open Source is that the highest handler is in fact the Preview so clicking on it will do nothing for let's say an image called a.jpg. Instead, we should determine based on the list of all available open handlers, what the best open source handler would be or if it should always be the editor.

If we take for example the case of an image file like a.jpg, the highest open handler would be the preview, but if we call the openSource based on its current implementation, we will also use the preview. In this case, we should get all open-handlers, and determine for the right context what the appropriate open-handler would be.

[vince-fugnitto]

Investigation of #5110