Improve how Dolphin works in read-only directories

This is a collection of some problems and proposed fixes when the current directory is not writable by the user.

Problem 1: No visual indication that the current directory is read-only.

Rationale: Plenty of actions are disabled in a read-only directory. So we'd better let the user know this early on, than he attempts to do something and fails.

Proposal: Reuse the red lock emblem we use on inaccessible directories, but put it at the end of breadcrumbs. Clicking on the emblem can bring up a dialog that explains the situation, and offer to open in admin mode.

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Problem 2: DnD into a read-only directory shows a "stop" sign, but on button release, the normal move/copy/link menu still shows, and choosing any item brings up a "Retry" dialog, but "Retry" won't help here.

Proposal: Don't show the menu. The "stop" sign should be clear enough to the user.

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Problem 3: Shortcuts like "Ctrl-V (Paste)" silently fails in a read-only directory, with no explanation to the user. (Actually it doesn't fail. The action is disabled, so is the shortcut.)

Proposal: These shortcuts should pop up a dialog explaining why they are disabled. (This might not be necessary if the fix to problem 1 is very noticeable, so the user won't erroneously press the shortcut at all.)