Add the CodeHacker#existsClass(String fullClass) method

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy: teach ImageJ 1.x about https in addition to http

This patch comes originally from fiji-compat; in the meantime,
another method in ListVirtualStack also has hard-coded http://
handling without appropriate https:// handling.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! LegacyInjector: Allow setting the application name
fixup! ij-legacy: teach ImageJ 1.x about https in addition to http
Introduce CodeHacker#dontReturnOnNull(String fullClass, String methodSig)

This method allows us to override ImageJ 1.x' check whether an Editor
instance could be created. We need this because we do not necessarily
want that Editor to be used, but maybe Fiji's, or -- soon -- ImageJ2's.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Allow CodeHacker#replaceCallInMethod to replace constructor calls, too

When passing '<init>' as methodName, replaceCallInMethod will() now
happily replace calls to constructors just as it was already handling
method calls.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Prepare IJ1Helper to open an editor for a patched ImageJ 1.x

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! ij-legacy: make sure no dialog is opened in headless mode
fixup! ij-legacy: add "Convert to 8-bit Grayscale" to File>Import>Image Sequence
Port fiji-compat's Script Editor hooks to ij-legacy

Fiji had to patch ImageJ 1.x in order to be able to use the fancy
script editor instead of the bland AWT-based one in ImageJ 1.x. We
will offer these extension points in ImageJ2 from now on, for use by
Fiji, but also by other 3rd-party software.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>