main: refactor signal handler and web view reloading

This commit introduces the following changes:

* wkline is a now a global variable in wkline.c, this is required to be
  able to reload the web view on SIGUSR1, and shouldn't be an issue
  regarding threading, as all widget/thread-related stuff is handled in
  widgets.c
* The web view context menu is now always disabled, and the web view can
  be reloaded by sending the SIGUSR1 signal instead of using the context
  menu
* Signal handlers are installed with sigaction() instead of signal()