Add watcher on sketch files

[facchinm]

Reloads sketch content if the Editor is not in foreground and something happens in the backing storage files.

Note that no confirmation dialog is displayed (same behaviour as SublimeText, differs from other IDEs)

Fixes #4551 and #5345

ONLY TESTED ON LINUX with /proc/sys/fs/inotify/max_user_watches = 524288

[per1234]

What about handling deletion of secondary sketch files (AKA "tabs") outside the IDE? If I delete one outside the IDE when using this test build 664 I get:

Could not createInput() for E:\Stuff\misc\electronics\arduino\temp\sketch_jul02a\foo.ino
java.io.FileNotFoundException: E:\Stuff\misc\electronics\arduino\temp\sketch_jul02a\foo.ino (The system cannot find the file specified)
	at java.io.FileInputStream.open0(Native Method)
	at java.io.FileInputStream.open(FileInputStream.java:195)
	at java.io.FileInputStream.<init>(FileInputStream.java:138)
	at processing.app.legacy.PApplet.createInput(PApplet.java:595)
	at processing.app.legacy.PApplet.loadStrings(PApplet.java:254)
	at processing.app.BaseNoGui.loadFile(BaseNoGui.java:605)
	at processing.app.SketchFile.load(SketchFile.java:256)
	at processing.app.EditorTab.reload(EditorTab.java:387)
	at processing.app.tools.WatchDir.processEvents(WatchDir.java:142)
	at processing.app.EditorTab$1.run(EditorTab.java:166)
	at java.lang.Thread.run(Thread.java:745)
Warning: Failed to reload file: "foo.ino"

If the watcher removed tabs that were deleted outside the IDE it would help fix #2072.

[ArduinoBot]

✅ Build completed.

Please test this code using one of the following:

⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6074-BUILD-704-linux32.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6074-BUILD-704-linux64.tar.xz
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6074-BUILD-704-windows.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6074-BUILD-704-macosx.zip
⬇️ http://downloads.arduino.cc/javaide/pull_requests/arduino-PR-6074-BUILD-704-linuxarm.tar.xz

ℹ️ The linuxarm build is still experimental and may not be always available.

[evaarnas]

It seems to work, unless I have unsaved changes in the Arduino IDE. That is, for "clean" files (all changes saved) it detects the external changes, but not for "dirty" files (with unsaved changes in the Arduino IDE).