viewer: add key passthrough mode

When trying to search in less, keys bound to viewer functions can't be
used as part of the search query, which makes the search useless.

Add a view::passthrough binding mode and a :toggle-key-passthrough
command go toggle in and out of that mode. By default, typing '/' in the
viewer is bound to enabling key passthrough and automatically inserting
'/', to easily enter "less" search mode. When in the passthrough mode,
all bindings are ignored by default. The default binds.conf only defines
a single keybinding in that mode: <Esc> to get back out. $ex is bound to
<C-x> to allow typing colons.

Signed-off-by: Kt Programs <ktprograms@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/msgview/toggle-key-passthrough.go

@@ -0,0 +1,35 @@
+package msgview
+
+import (
+	"errors"
+
+	"git.sr.ht/~rjarry/aerc/widgets"
+)
+
+type ToggleKeyPassthrough struct{}
+
+func init() {
+	register(ToggleKeyPassthrough{})
+}
+
+func (ToggleKeyPassthrough) Aliases() []string {
+	return []string{"toggle-key-passthrough"}
+}
+
+func (ToggleKeyPassthrough) Complete(aerc *widgets.Aerc, args []string) []string {
+	return nil
+}
+
+func (ToggleKeyPassthrough) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) != 1 {
+		return errors.New("Usage: toggle-key-passthrough")
+	}
+	mv, _ := aerc.SelectedTab().(*widgets.MessageViewer)
+	keyPassthroughEnabled := mv.ToggleKeyPassthrough()
+	if keyPassthroughEnabled {
+		aerc.SetExtraStatus("[passthrough]")
+	} else {
+		aerc.ClearExtraStatus()
+	}
+	return nil
+}