Refresh the available list of libraries whenever the Import Library menu is expanded

[remoun]

This way the Arduino IDE doesn't need to be restarted whenever a user drops a new library directory into Documents/Arduino/libraries or the sketchbook's libraries folder.

Note: I'm not sure how slow the rebuildImportMenu call can get if there are tons of libraries, but I expect it won't be so bad, since it's just a few stat() calls. Perhaps we could guard it with a setting (preferably defaulting to true [rebuild]) if it proves too slow.

[matthijskooijman]

Another thing that comes to mind is that IIRC the IDE sometimes shows a popup when a library is invalid (or does it only print to stderr?). In this case, you might never get to open the import menu if a popup is shown (which probably prevents you from using the import menu)? Not sure if this is actually the case, but it's at least something to double-check.

I had a quick look at the commits, they look good to me.

[remoun]

Good catch. Looking at the code, it doesn't seem too bad, only when filenames are invalid*.

I also verified it; a dialog does pop up, but that doesn't stop the menu from appearing (without the bad lib names). So it's just a bit noisy, but it's (a) a corner case [when filenames are invalid], (b) it's clear about which library names are invalid, so (c) it's easy to remedy that error condition.

• An error dialog happens when a library fails to import, but that only happens

[remoun]

I added a note about it (see 25acf30).

[ffissore]

Sorry for the late reply. A side effect of introducing Boards/Lib managers with IDE 1.6.2 is that both cores and libraries get refreshed as soon as one of the Manager gets closed. Plus, filesystem monitoring and background library parsing are expensive, on windows especially because of antivirus software.