Added support for only launching a single instance

Since it probably defeats the purpose to be running multiple pomodoro
timers, I've added support to only launch one instance of the program.
This resolves Issue #43

Cargo.toml

@@ -18,6 +18,7 @@ hex_color = { version = "2.1.0", features = [ "serde" ] }
 walkdir = "2.4"
 etcetera = "0.8.0"
 directories = "5.0.1"
+single-instance = "0.3.3"
 
 
 [target.'cfg(windows)'.dependencies]

src/main.rs

@@ -12,6 +12,7 @@ use hex_color::HexColor;
 use serde::{Deserialize, Serialize};
 use serde_json::json;
 use settings::{GlobalShortcuts, JsonSettings};
+use single_instance::SingleInstance;
 use slint::{Color, ModelRc, Timer, TimerMode, VecModel};
 use std::{
     fs::File,
@@ -243,6 +244,12 @@ impl Main {
 }
 
 fn main() -> Result<()> {
+    let instance = SingleInstance::new("org.vadoola.tomotroid").unwrap();
+    assert!(instance.is_single());
+    if !instance.is_single() {
+        return Err(anyhow::anyhow!("Only one instance of Tomotroid is allowed to run"));
+    }
+
     //TODO: I'm not seeing an obvious way to mimic the Pomotroid behavoir
     //where it just minimizes or restores by clicking the tray icon
     //because I don't see any way to capture when the tray icon is clicked