Make levels safer

It is now impossible to have any level set to something below 5% of
the battery and above 50%. If a lower level is set higher than its
higher level, the program will attempt to lower the level by 1%
without going under 5%.

src/battery_state.rs

@@ -29,8 +29,8 @@ pub const DEFAULT_REFRESH: u64 = 5;
 impl BatteryState {
     pub fn new(
         low_level: f32,
-        very_low_level: f32,
+        mut very_low_level: f32,
-        critical_level: f32,
+        mut critical_level: f32,
         refresh_rate: u64,
         verbosity: VerbosityLevel,
     ) -> battery::Result<Self> {
@@ -48,6 +48,14 @@ impl BatteryState {
             }
         };
 
+        let low_level = low_level.min(50_f32).max(5_f32);
+        if very_low_level > low_level {
+             very_low_level = f32::max(low_level - 1_f32, 5_f32)
+        };
+        if critical_level > very_low_level {
+            critical_level = f32::max(very_low_level - 1_f32, 5_f32)
+        };
+
         Ok(Self {
             manager,
             battery,