feat: Add pane sync and prefix key status indicators

The indicators used can be customized:

- For the prefix key indicator, set `@themepack-prefix-key-on-format`
and `@themepack-prefix-key-off-format` options. The off format must be
the same length as the on format to avoid weird behavior.
- For the pane sync indicator, set `@themepack-pane-sync-on-format` and
`@themepack-pane-sync-off-format` options. The off format must be the
same length as the on format to avoid weird behavior.

test/powerline.go

@@ -0,0 +1,57 @@
+package test
+
+var powerlinePrefixKeyStatus = `#{?client_prefix,#{@themepack-prefix-key-on},#{@themepack-prefix-key-off}}`
+var powerlinePaneSyncStatus = `#{?pane_synchronized,#{@themepack-pane-sync-on},#{@themepack-pane-sync-off}}`
+
+var powerlineColors = map[string]struct {
+	filename string
+	color1   string
+	color2   string
+	color3   string
+}{
+	"blue": {
+		color1: "colour24",
+		color2: "colour33",
+		color3: "colour31",
+	},
+	"cyan": {
+		color1: "colour39",
+		color2: "colour81",
+		color3: "colour75",
+	},
+	"gray": {
+		color1: "colour245",
+		color2: "colour250",
+		color3: "colour245",
+	},
+	"green": {
+		color1: "colour100",
+		color2: "colour190",
+		color3: "colour107",
+	},
+	"magenta": {
+		color1: "colour125",
+		color2: "colour127",
+		color3: "colour129",
+	},
+	"orange": {
+		color1: "colour130",
+		color2: "colour166",
+		color3: "colour130",
+	},
+	"purple": {
+		color1: "colour90",
+		color2: "colour129",
+		color3: "colour141",
+	},
+	"red": {
+		color1: "colour88",
+		color2: "colour160",
+		color3: "colour124",
+	},
+	"yellow": {
+		color1: "colour227",
+		color2: "colour227",
+		color3: "colour229",
+	},
+}